Source: NHI course on Travel Demand Forecasting, Ch. 8 (

152054A)

Objectives

• Define external trip purposes • Identify the trip distribution inputs for each external trip purpose • Identify the trip distribution outputs for each external trip purpose • Apply a Fratar distribution to a sample data set

Terminology

• Study area boundary location – Encompass current urban area plus forecasted urban area (20-25 years) – Extend boundary to include commute trips into the area for work, etc. (practical?) – Have no more than 15% of VMT from trips that begin or end outside of study area – Cover non-attainment area • Trip length characteristics – Trip characteristics differ depending on whether the trip maker lives within or outside of the study area (so different friction factor curve, right? Really?)

IE is usually modeled as a separate trip purpose Q: what about II trips made by IE travelers?

Key Concepts

Inputs and Outputs

• Inputs for external-internal trips (from external survey) – Productions for each external station TAZ – Trip length frequency distribution – Travel time impedance matrix – Scaled NHB attractions by TAZ • Inputs for external-external trips – Counts at each external station TAZ – Origin and destination trip table (base-year) • Outputs – 24-hr external-internal trip table (P&A) – 24-hr external-external trip table (O&D)

Data Collection

• Vehicle Counting and Classification – 24-hr traffic counts for each external station TAZ (to expand survey data) – Manual counts used to correct automatic counts (vehicle classification)

External Station Survey Methodology… • Have an approved traffic control plan • Survey in outbound direction (why?) • Move vehicles out of traffic stream • Conduct interviews or give postcards • Interview in daylight hours only • Survey maximum number of vehicles • Sample all high volume locations • Take random sample of low volume locations

Data Elements to Collect

• Number of persons in vehicle • Highway name/number (for entry and departure) • Time entered study area • Time departed study area • Address of last stop in study area • Trip purpose of last stop and next stop • Home address (to see if they live in the study area)

Data analysis

1. Expand survey data (count to survey ratio) 2. Separate trips between external internal and external-external 3. Develop TLFD for external-internal trips (to calibrate gravity model) 4. Distribute external-internal trips – Use scaled NHB trips as a proxy 5. Apply the gravity model

Data Analysis

Diagonal Always zero

Forecasting Future Year Trip Tables

• Forecast total volume for each external station • Assume split between external-internal and external-external is same as base year • Forecast external-internal trip tables • Forecast external-external trip tables

Fratar Example